In recent years, vehicle tires having a low fuel consumption are eagerly desired from the viewpoint of protection of environment and conservation of natural resources. Further, good wet-grip property and high abrasion resistance are required for the tires from a viewpoint of safety and durability, respectively.
A silica-incorporated rubber composition exhibits reduced heat build up as compared with the conventional rubber composition having carbon black incorporated therein. Therefore, a silica-incorporated rubber composition is suitable for the production of tires with a low fuel consumption.
Silica generally has a poor affinity for most conventional rubbers. Therefore most rubbers having silica incorporated therein exhibit poor processability and are not satisfactory in heat build up and abrasion resistance. To obviate this problem, a silane coupling agent is often used in combination with silica. However, a silane coupling agent improves the abrasion resistance sometimes only to a very minor extent as compared with the incorporation of carbon black. Further a silane coupling agent is expensive and the incorporation thereof in a sufficient amount is costly.
Attempts of modifying a rubber to enhance its affinity for silica have been made. For example, a silica-incorporated rubber composition is proposed in Japanese Unexamined Patent Publication No. H10-7702 which is made by a diene polymer rubber is lithiated with an organic lithium compound and then the lithiated diene polymer rubber is allowed to react with s silicon-containing compound. A rubber composition comprising a diene polymer containing a silanol group, and a specific carbon black having silica fixed on the surface thereof is proposed in Japanese Unexamined Patent Publication No. H10-316800. The above-proposed rubber compositions give a rubber vulcanizate exhibiting good reduced heat build up, but, the silica-incorporated unvulcanized rubber composition has poor processability and the rubber vulcanizate tends to exhibit poor balance between wet-grip property and abrasion resistance.
A rubber composition comprising a polyorganosiloxane-modified diene polymer rubber and silica is proposed in Japanese Unexamined Patent Publication No. H9-110904. The polyorganosiloxane-modified diene polymer rubber is made by allowing a diene polymer having an active alkali metal terminal, which is prepared by polymerization using an alkali metal polymerization initiator, to react with 0.1 to 2 moles, per mole of the alkali metal polymerization initiator, of a polyorganosiloxane having a specific functional group.
A rubber composition comprising a silsesquioxane-modified diene polymer rubber and silica is proposed in Japanese Unexamined Patent Publication No. 2002-80534. The silsesquioxane-modified diene polymer rubber is made by allowing a diene polymer having an active alkali metal terminal, which is prepared by polymerization using an alkali metal polymerization initiator, to react with 0.1 to 1.5 moles, per mole of the alkali metal polymerization initiator, of a silsesquioxane compound having a polyhedron structure.
The above-proposed polyorganosiloxane-modified diene polymer rubber and the silsesquioxane-modified diene polymer rubber give a rubber vulcanizate having well balanced reduced heat build up and wet-grip property, but, its unvulcanized silica-incorporated rubber composition has poor processability and the rubber vulcaniate tends to have poor abrasion resistance.
Further in the case when a rubber is loaded with carbon black in combination with silica to impart an antistatic property, an abrasion resistance and driving controllability as tire, balance between reduced heat buildup and wet-grip property is not enhanced to the desired extent.